1. Field of the Invention
The present invention relates to a coaxial-line low-pass filter for use in a high-frequency transmission circuit.
2. Description of the Related Art
A coaxial-line low-pass filter in the related art is described below with reference to FIG. 7.
FIG. 7 is a side cross-sectional view partially showing the low-pass filter taken along a plane which is parallel to the signal propagation direction and which includes the central axis of an inner conductor.
The low-pass filter shown in FIG. 7 includes a tubular outer conductor 7 with a substantially uniform inner diameter, an input/output unit 10, and an inner conductor formed of high-impedance portions 701 and low-impedance portions 702.
In FIG. 7, len701 indicates the length (axial length) of the high-impedance portion 701 of the inner conductor in the signal propagation direction; len702 indicates the length (axial length) of the low-impedance portion 702 of the inner conductor in the signal propagation direction; w701 indicates the diameter (axial diameter) of a plane of the high-impedance portion 701 of the inner conductor which is vertical to the signal propagation direction; w702 indicates the diameter (axial diameter) of a plane of the low-impedance portion 702 of the inner conductor which is vertical to the signal propagation direction; Z indicates input impedance of the low-pass filter; Zhi indicates characteristic impedance of the high-impedance portion; and Zlow indicates characteristic impedance of the low-impedance portion.
The inner conductor comprises a plurality of cylindrical members formed of a predetermined number of high-impedance portions 701 and a predetermined number of low-impedance portions 702 which are alternately connected with each other. The high-impedance portions 701 and the low-impedance portions 702 of the inner conductor are connected with each other so that the central axes of the high- and low-impedance portions 701 and 702 are aligned in a line. The inner conductor is placed in the outer conductor 7 so that the central axis of the inner conductor matches the central axis of the outer conductor 7.
The high-impedance portions 701 of the inner conductor have the same length (axial length) len701, and the same width (axial diameter) w701, thus allowing characteristic impedance Zhi to be constant thereacross. Likewise, the low-impedance portions 702 of the inner conductor have the same length (axial length) len702, and the same width (axial diameter) w702, thus allowing characteristic impedance Zlow to be constant thereacross.
The input/output unit 10 having input impedance Z is connected to the high-impedance portion 701 at an end of the inner conductor.
The high-impedance portions 701 function as inductors, while the low-impedance portions 702 function as capacitors. A low-pass filter including a plurality of LC resonator circuits connected in series is thus achieved.
Such a low-pass filter in the related art has problems.
In the low-pass filter shown in FIG. 7 in which the inner diameter of the outer conductor 7 is uniform, resonance of one-half wavelength of a transmission signal is produced in the high-impedance portions 701 of the inner conductor which has a smaller axial diameter. This causes spurious resonance peaks in the attenuation region of the low-pass filter, resulting in an undesired attenuation characteristic. If a plurality of high-impedance portions having the same axial length and the same axial diameter are used to form the filter, the positions of spurious resonance peaks in the high-impedance portions coincide with each other, thus causing overlapping spurious responses to induce higher spurious resonance peaks.
In order to reduce such spurious resonance, a low-pass filter has been proposed in which high-impedance portions have different axial lengths and axial diameters. Specifically, different axial lengths and widths of the high-impedance portions allow spurious resonance peaks to be produced at different frequencies in the high-impedance portions so as to disperse spurious resonance peaks. This mechanism prevents overlapping spurious resonance peaks, which does not affect an attenuation characteristic.
In such a low-pass filter, if the high-impedance portions have different axial lengths while maintaining constant characteristic impedance, the axial diameters of the high-impedance portions must differ from each other in the case where the inner diameter of the outer conductor is uniform. That is, the axial diameter of a high-impedance portion must be reduced in order to make the axial length thereof shorter, and the axial diameter of a high-impedance portion must be increased in order to make the axial length thereof longer. This does not cause a problem if the length of the low-pass filter can be freely designed. However, if the length of the low-pass filter is restricted, the filter has a mixture of a high-impedance portion with small axial diameter and a high-impedance portion with great axial diameter.
Typically, a lathe or the like is used to cut a material having a certain thickness into the shape of an inner conductor of a low-pass filter.
Thus, an inner conductor having too small an axial diameter would be off-centered during a cutting process, and is difficult to cut, thus increasing the production cost or causing defective products. A finished inner conductor would also have lower resistance to vibration or shock.
For example, in a multistage low-pass filter in which the length between input/output units at both ends thereof is 100 mm, and the diameter of a low-impedance portion is about 20 mm, the diameter of a high-impedance portion must be 2 mm or greater in order to facilitate the cutting process for the inner conductor. With the structure of the above-described low-pass filter, however, the width of a high-impedance portion can be less than 2 mm in design.